“Night sky” is intended to mean a part of space visible above the horizon from an arbitrary point of the Earth or from the air and which no longer receives sunlight on account of its position with respect to the Sun.
Generally, a device for detection or imaging of an aerial target in a night sky comprises a thermal infrared camera.
The infrared camera makes it possible to highlight multiple phenomena by picturing as an image the value of the temperatures of the observed objects or their signature associated with infrared emission lines.
The infrared camera senses through the atmosphere radiation emitted by the objects. A radiometric system converts the radiation power into digital or analog signals. The latter are optionally transcribed into illuminations, luminances or else into temperatures by a computer and transformed into luminous points on a screen. The infrared camera uses one or more spectral bands corresponding to the infrared and more particularly a first spectral band extending over a range of wavelengths lying between 3 and 5.5 micrometers called the IR2 band or “Mid Wavelength InfraRed” or MWIR region, or a second spectral band extending over a range of wavelengths lying between 8 and 12 micrometers called the IR3 band or “Long Wavelength InfraRed” or LWIR region.
Thermal cameras are used in numerous applications such as the detection of weak points of the insulation of a building or the detection of victims during fires. Infrared cameras are also used during nighttime military operations for the detection, recognition and identification of targets, notably.
The image of an aerial target in a night sky obtained with the aid of an infrared camera comprises the target but also hot combustion gases issuing from the propulsion units of the target and called “propulsion plumes”. The combustion gases comprise a significant proportion of carbon dioxide exhibiting a very strong emission band in an MWIR region and more particularly a wavelength band centered on 4.2 micrometers. The presence of carbon dioxide at high temperature renders a target's combustion gases visible on an image obtained with the aid of an infrared camera operating in band 2 or MWIR.
The image obtained can be delivered in black and white with a gray level proportional to the illumination of the source if it is not resolved or to its luminance if it is angularly resolved. Another way of delivering these images can be achieved by a color coding.
This IR2 band of the MWIR is therefore well suited to carrying out the detection of flying targets at long range. On the other hand, depending on the orientation of the target with respect to the infrared camera, the identification of the target may turn out to be difficult because of the presence of these propulsion plumes which may mask the real contours of the target. Indeed, the strength of the emissions of the signature of the target itself is often much lower than the strength of the emissions of the propulsion plumes.